The present invention relates to a method for forming a green tread rubber with which it is possible to form, at high quality, a green tread rubber of double-layered structure comprised of a base rubber layer and a cap rubber layer, and a pneumatic tire formed by using the green tread rubber.
It is common practice with pneumatic tires to employ a double-layered structure for the tread rubber comprised of an inner base rubber layer and a cap rubber layer outside thereof for improving durability, steering stability, grip performances and tire performances such as low rolling resistance.
Styrene/butadiene type rubber that exhibits superior aging resistance, heat resistance, wear resistance and wet skid resistance is employed as the cap rubber layer. On the other hand, natural rubber type rubber that exhibits repulsive elasticity and low heat-generating properties is employed as the base rubber layer.
For instance, in case of high-performance tires that exhibit superior high-speed running performances and winter tires that exhibit superior on-snow performances employ styrene/butadiene rubber as a rubber component for the cap rubber layer. With this arrangement, properties such as the above heat resistance, wear resistance and wet skid resistance are improved. In case of winter tires, mixed rubber of natural rubber and butadiene rubber exhibiting superior wear resistance and low temperature properties is employed as the rubber component of the cap rubber layer.
In this respect, it is assumable to set a complex elastic modulus E2* of the base rubber layer to be larger than a complex elastic modulus E1* of the cap rubber layer. With this arrangement, it is expected to improve the tread rigidity, to improve the steering stability when performing high-speed running or the steering stability on icy and snowy roads or when running on general roads.
On the other hand, green tread rubber prior to vulcanization molding for forming such tread rubber is conventionally manufactured in the following manner. More particularly, as illustrated in FIG. 5(A) in conceptual form, a molded body c of double-layered structure is successively extruded from a rubber extruder a. The molded body c of double-layered structure is cut into constant size to meet a peripheral length of a molding drum D. The cut rubber bodies c1 of constant size are once stored by being accumulated in a multi-staged manner on a storage carrier e. When molding a tire, the storage carrier e is transferred to a tire molding line for supplying the rubber extruded bodies c1 to the molding drum D. The rubber extruded bodies c1 are wound around the molding drum D by a single round. End portions f, f in a circumferential direction are mutually abutted and joined. With this arrangement, a green tread rubber t for green tire molding is formed in an annular shape. In this respect, reference g in the drawing denotes a cooling line.
However, in such a conventional method, adhesive force between the end portions f, f tends to fall short. Owing to this fact, an opening tends to be generated at joint portions j of the green tread rubber t in the course of manufacturing a tire. Particularly in case the base rubber is made highly elastic as in the above-described case, the adhesiveness of rubber is apt to be degraded accompanying the high elasticity. The lack in adhesiveness at the base rubber layer originates in and promotes the opening at the cap rubber layer. This accordingly leads to degradations in yield ratio and tire quality. When using styrene/butadiene rubber, such rubber itself tends to be inferior in adhesiveness when compared to natural rubber or the like, and the tendency of occurrence of opening is stronger.
Thus, when joining the end portions f, f, it might happen at the cap rubber layer t1 that the end portions f, f are not sufficiently adhered as illustrated in FIG. 5(B). It might thus happen that the joint portions j open in the course of manufacturing a tire. It might also happen that the joint portions j comprise weak points from which cracks occur. In case of styrene/butadiene type rubber, it exhibits easily shrinking properties. Dimensional changes (reductions in length) of the rubber extruded bodies c1 during storage are accordingly large, which promotes the opening or occurrence of cracks. Such opening and occurrence of cracks become particularly remarkable when the blending amount of styrene/butadiene rubber in styrene/butadiene type rubber is increased to not less than 80 parts by mass for the purpose of improving the tire performance.
The inventors of the present invention have thus suggested forming only the base rubber layer by means of a rubber extruded body from a rubber extruder while the cap rubber layer, which exhibits inferior adhesiveness, is formed through a so-called strip-wind method in which a tape-like rubber strip is overlapped and successively wound around the base rubber layer in a circumferential direction and in a spiral manner.
According to this method, it will be possible to reliably prevent opening or cracks at the cap rubber also when the blending amount of styrene/butadiene rubber is high and to improve the yield ratio and the tire quality. In the green tread rubber, the rubber composition of the cap rubber layer is changed in accordance with types of tires and other factors. The method in the prior art increased intermediate stock, since the cap rubber layer and the base rubber layer are integrally extruded whereby it will be required to change the rubber and to newly form double-layered rubber extruded bodies c1. However, according to the present invention, the cap rubber layer and the base rubber layer are separately formed so that the base rubber layer can be standardized to some extend or partially, and it will be possible to reduce the volume or types of intermediate stock, since it may be required to change rubber of, for instance, the rubber strip only.
In this respect, the inventors of the present invention have suggested, in published patent application 2002-127718, winding a soft rubber strip onto an inner layer in case of carcass rubber rather than tread rubber. However, this suggestion concerns a technique related to soft carcass rubber that is employed at carcass portions. It is accordingly assumed that a soft rubber strip having a required specified hardness, degree of tension, and Mooney viscosity is employed.